Fecal blood testing remains the only non-invasive approach to colorectal cancer screening. However, neoplasms bleed intermittently and many other sources of gastrointestinal blood loss exist making these widely used tests inherently ambiguous with common false-negative and false-positive results. A more sensitive and specific stool screening marker would translate into more efficient use of our limited health care resources and more effective colorectal cancer prevention, especially if pre-malignant adenomas were better detected. There is a compelling biological rationale to target tumor-specific markers on colonocytes, which are continuously shed into stool. The overall objective of this proposal is to validate a stool assay system based on immunodetection of the neoplasm-derived colonocyte markers, Ag 33.28 and MUC-1, which have been shown immunohistochemically to highly discriminate colorectal neoplasia from normal mucosa. These markers will be detected by selective immunolabeling of whole fecal colonocytes isolated by density gradient centrifugation. Marker-positive colonocytes will be quantified by immunocytochemistry and flow-cytometry. In the first phase of this study, the effects of critical stool variables (storage, hydration, etc) and lesion variables (size, site, stage, etc) on assay results will be rigorously evaluated from a subject group with known colorectal neoplasia. Solid-phase immunocapture will be explored as an alternative approach to colonocyte isolation, and sandwich immunoassay of Ag_33.28 and MUC-1 from crude fecal lysates will be examined. In the second phase, the sensitivity and specificity of the optimized colonocyte antigen assays for detecting clinically important colorectal neoplasia (cancers and large adenomas) will be determined in 2000 higher-than-average risk persons scheduled for colonoscopy, which will serve as a gold standard. In an effort to generate the most relevant clinical data, the performance of colonocyte antigen assays will be prospectively compared in blinded fashion to that of fecal blood tests--conventional fecal hemoglobin tests (Hemoccult, HemoQuant) and the recently developed calprotectin test. Data analyses will address the impact of demographic, dietary, medication, and other patient co-variates for each assay. Should the colonocyte marker assays outperform fecal blood tests, evaluation of their application in the general screening and in various surveillance settings would be strongly supported.